In recent years, a compact copying machine which is capable of producing a large number of copies at high speed while maintaining high image quality is demanded. Among characteristics of a toner used for such a copying machine, low-temperature fixing property and heat-resistant storage stability are in a trade-off relationship. There is a problem that reducing thermal properties such as glass transition temperature and melt viscosity to achieve low-temperature fixing property degrade heat-resistant storage stability.
As an improvement of the low-temperature fixing property, for example, a binder resin for a toner including a non-crystalline polyester resin and a crystalline polyester resin with significantly improved low-temperature fixing property compared to the non-crystalline polyester resin is proposed (see PTL1). However, when the crystalline polyester resin and the non-crystalline polyester resin are used in combination, a transesterification reaction occurs during melt-kneading due to similar composition of the resins. Thus, high-crystallinity of the crystalline polyester resin cannot be maintained, and heat-resistant storage stability of the toner tends to decrease. Also, addition of the fresh crystalline polyester resin complicates a system, and as a result, problems of side effects such as degradation of granulation and carrier pollution tend to occur.
Also, binder resins for a toner composed of a crystalline polyester resin and a styrene-acrylic resin are proposed (see PTL2 and PTL3). In these proposals, storage stability of a toner at a low temperature and fixing property of a toner at low speed are evaluated, but further improvements in low-temperature fixing property and heat-resistant storage stability are desired.
Also, as a binder resin, a toner including a crystalline polyester resin including a structure represented by —OCOC—R—COO—(CH2)n— (where, in the formula, R represents a straight-chain unsaturated aliphatic group having 2 to 20 carbon atoms, and n represents an integer of 2 to 20) at a rate of 60% by mole of the total ester bonds in the entire resin structure is proposed (see PTL4). With the toner of this proposal, low-temperature fixing property improves, but improvement of heat-resistant storage stability is not studied at all.
As a further improvement of low-temperature fixing property, coupled with an addition of a crystalline polyester resin, lowering melt viscosity by reducing the weight-average molecular weight of a non-crystalline polyester resin has been attempted. However, reduction of the weight-average molecular weight increases a presence of an oligomer component derived from the non-crystalline polyester resin. This oligomer component has a high proportion of polar groups per unit structure and is water-soluble. Thus, when toner particles are dispersed in an aqueous medium during a manufacturing process, the oligomer component eludes into the aqueous medium. A toner as a powder thereof in the end has high thermal properties such as glass transition temperature and melt viscosity as a whole, and there is a possibility that the toner does not develop low-temperature fixing property. Also, the oligomer component derived from the non-crystalline polyester resin has a glass transition temperature below a normal temperature and is adhesive below a room temperature. Thus, the oligomer component may promote the adhesion among toner particles if it remains on a surface of the toner particles.
Thus, a production of a toner with addition of a crystalline polyester resin, reduction of a weight-average molecular weight of a non-crystalline resin and reduction of a content of an oligomer component which satisfies superior low-temperature fixing property as well as heat-resistant storage stability is desired.